A magnetic recording device (HDD), which performs recording and reproduction (write and read) of information and is mainly used for computers, has large capacity, low price, good data access speed, reliability of data-storage and the like, and therefore, the scope of application of the magnetic recording device has been expanding gradually in recent years. The magnetic recording device is used in various fields, such as household videotape recorders and players, an audiovisual apparatuses, and in-vehicle navigation systems. In accordance with such an increase of usable range of HDD, the demand for obtaining the record capacity having high density is also increased, and as a result, development of HDD for obtaining higher density is thriving in recent years.
An in-plane magnetic recording system is a system used for the magnetic read/write apparatus which is commercially available at present. In the system, magnetic crystal grains, which are comprised in a magnetic recording layer used for recording the information, have a magnetic easy axis thereof in parallel to the substrate. Here, the magnetic easy axis means an axis to which the direction of magnetization tends to be oriented easily. For example, in the case of a Co-based alloy, a magnetic easy axis thereof is the “c” axis of the hcp structure of Co. The in-plane magnetic recording medium has an ability that read/write characteristic thereof degrade due to so-called thermal fluctuation effects, wherein a magnetization reversal unit diameter of the magnetic layer is too decreased corresponding to decreased record bit which is made small in order to increase a recording density, and as a result, the information recorded therein is thermally lost. Furthermore, when the record density is increased, noise which is generated from the medium tends to increase under the influence of the demagnetizing field generated in the boundary area between record bits.
On the other hand, there is a so-called perpendicular magnetic recording system in which a magnetic easy axis in a magnetic recording layer is oriented to a direction which is nearly perpendicular to a substrate. In this system, the influence of demagnetizing field between record bits is small even while recording density of the medium is increased, and the system is magnetostatically stable even when recording density is increased. For this reason, the perpendicular magnetic recording system has been of great interest in recent years as a technology which can be used instead of the in-plane recording system. In general, the perpendicular magnetic recording medium comprises a substrate, an orientation controlling undercoating layer for obtaining the orientation of a magnetic recording layer, the magnetic recording layer which is formed with a hard magnetic material, and a protective layer which protects the surface of the magnetic recording layer. A soft magnetic back layer may be provided between the substrate and the undercoating layer, wherein the back layer functions for concentrating the magnetic flux which is generated from a magnetic head at the time of recording.
The perpendicular magnetic recording medium is also required to realize a low noise while maintaining a heat stability, in order to establish a high record density thereof.
As a method generally used for decreasing noise, there is a method in which size of the magnetic crystal grains itself included in the recording layer is made fine. For example, CoCr based magnetic recording layer which is widely used in general is explained below. Magnetic grains for the magnetic layer are made fine, due to a segregation of nonmagnetic Cr in the grain boundary wherein the segregation is caused by adding Ta and B or by heating to a suitable temperature. However, in the case of a perpendicular magnetic recording medium, magnetic interaction between grains in the perpendicular magnetic recording medium is decreased insufficiently, since it is not sufficient to achieve the fine magnetic grains caused by Cr segregation and the spatial separation between magnetic crystal grains is imperfect. Therefore, the perpendicular magnetic recording medium has a problem that the transition noise between record bits decreases insufficiently.
As a method for reducing the aforementioned magnetic interaction, there is a method in which SiO2 is added to a recording layer to obtain a magnetic recording layer having a granular structure wherein magnetic crystal grains were enclosed with an SiO2 additive. For example, please refer to Japanese Unexamined Patent Application, First Publication No. 2002-83411.
Moreover, there is a method in which TiO2 is added to a recording layer to obtain a magnetic recording layer having a granular structure in which magnetic crystal grains were enclosed with the TiO2 additive. For example, please refer to Japanese Unexamined Patent Application, First Publication No. 2001-43526.
However, there is a problem in that SiO2 has a slow diffusion velocity in a film and therefore it cannot fully deposit in the magnetic crystal grain boundary, and for this reason, a part of SiO2 which cannot deposit forms a supersaturated solid solution with the magnetic crystal grains, and a crystallinity and an orientation of the magnetic crystal grains cause disorder or distortion, and as a result, the signal to noise ratio (SNR) of read/write characteristic (R/W) is reduced.
Moreover, there is a problem in that TiO2 has high heat stability and therefore it does not fully deposit at the magnetic crystal grain boundary, and for this reason, a part of TiO2 forms a supersaturated solid solution with the magnetic crystal grains, and a crystallinity and orientation of the magnetic crystal grains causes disorder or distortion, and SNR of the R/W characteristic is reduced.